A cationic electrodeposition paint has been widely employed in automobile industry as a primer for automotive bodies, because of good corrosion resistance.
The cationic electrodeposition paint is typically an aqueous dispersion which is prepared by dispersing a vehicle component consisting of a cationic resin (e.g. amine-modified epoxy resin, and the like) and a curing agent or a crosslinking agent (e.g. blocked polyisocyanate, and the like) with a pigment paste which contains pigment dispersed in a pigment dispersing resin, in an aqueous medium. Cationic electrodeposition is generally conducted by preparing an electrodeposition bath using the above mentioned electrocoating composition, putting an article to be coated therein and applying electric current as the article being cathode to deposit an electrocoated film on the surface of the article, followed by heating to cure the film.
It is desired that the resulting cationic electrocoated film has good surface smoothness without surface defects such as cratering and cissing, and the like, and also has sufficient adhesion when applying an intermediate coating and/or a top coating thereon, which does not adversely affect on smoothness and gloss of the top coating.
It is believed that the surface defects are caused by an organic compound showing low surface tension, an impurity in the pigment, a dust in an oven employed during the baking process or an oil sputtered from a seam of the automotive bodies, etc. Particularly, the oil may bump during the baking process to form craters on the surface of the coating. Problems referred to cratering or cissing would be eliminated if origins of contamination such as the dust are removed from the surface of the coating. However, it is significantly difficult to industrially carry out the above elimination.
Many approaches to prevent the surface defects had been suggested, including, for example, increasing a weight ratio of the pigment/base resin in the cationic electrodeposition paint or polymerizing the base resin to reduce flowability of the coating during the baking. These approaches, however, had disadvantages, such as poor smoothness of the resulting coating, which had been caused by the increase of a viscosity of the paint.
In order to prevent the above surface defects on the coating, the electrocoating composition generally contains lead compound. It is known that the lead compound melts during the baking process after coating the paint and accelerates curing of the base resin with a crosslinking agent, which makes smooth curing possible. However, since the lead compound may cause environment pollution when producing and/or wasting the paint after used, there has been recently desired a paint free from the lead compound.
For obtaining a cationic elecrocoating composition which does not contain the lead compound, an additive for inhibiting craters (such as cissing inhibiting agent, leveling agent or anti-crater agent, having the same function) has been reported. Examples of the additive include one that is formed from polyoxyalkylenepolyamines as described in Japanese Patent Kokai Publication No. Hei 2 (1990)-4826 and fluorine-containing copolymers as described in Japanese Patent Kokai Publication No. Hei 6 (1994)-184471, and the like.
The additive, however, when adding in an amount enough to inhibit cratering or cissing, may increase the viscosity of the electrocoating composition or reduce adhesion with PVC sealer, an intermediate coating or a top coating.